I was looking around on craigslist and it seems like there are a lot of civic hatches and crx engines lying around. I was thinking about dropping a crx engine into a civic eg or vx. My question, would I need to rebuild the engine if I was really aiming for mpg? I guess I could do a compression test to see if the rings will hold any compression. Could I leave the internals alone and just replace spark plugs and injectors?

You het what you pay for. Throw a unknown motor in and ????? It's a crap shoot. If you have the motor or a motor out unless you are super broke you just rebuild it. Have it taken to a machine shop and new pistons rings valve job ect. The only way I wouldn't do this if it was a 500$ dollar beater and you could hear the engine run before removal and like you Said a vacuum and compression test. you generally don't need injectors.

I will be doing this for cheap. I have a hard time putting money into a car that will be ravaged by the winter salt. I'm still torn between building a decent car and watching it rust or build a low budget car with some rust.

A CRX's engine is not going to give you a boost in mileage over the stock civic VX engine, people have put VX engines in CRX's and gotten a boost in mileage but I don't think you will see that by taking an older engine from a light weight car and putting it in a newer heavier car.

There are a lot of the engines around partly because they are less likely to wear out then to have the rest of the car rust out, so finding a good shell without an engine does happen but is rare and most of the time that it does happen it's had so much of the wiring and everything else stripped out of it to make it in to a race car that you are going to spend a bunch of money just to get it running.

So there is a good chance that when you find a car that you want to put an engine in to, that it will already have an engine in it that will outlast the body.

The VX is the best of the Civics aside from maybe an HX but I believe the CRX (especially the HF) is both lighter and has potential for a lower CdA than any 90's Civic. The VX and HX came with more powerful but also more sophisticated engines capable of lean burn which gives them an edge in economy over the D15B6 in the HF.

Ideally, you'd want a CRX with a VX or HX engine. Going with a CRX you give up backseats though.

So I found a crx HF and I think I've found a D15Z1. The only issue is that the CRX has "a lot of rust". It doesn't look to be rusted through anywhere but has a bit of surface rust I am actually okay with going through and re-welding body pieces and such. (I've been looking for an excuse to learn how to weld). Its a $600 HF and it will not look pretty so learning to weld on its body panels won't be a bit deal. I could also merge this all into aero stuff. I'm just worried about structural pieces being rusted through. I know the Geo's have issues with a-arms. Do CRX's have structural rust issues with rust?

I did look at that article but it doesn't seem like there are any major structural issues. I guess the a-pillars would be a pain to fix or door issues but those would be pretty obvious iI believe. It is a 1990 crx so it shouldn't have the issues of the panhard rod that exists in the 1st gen.